This revised competing renewal involves work previously funded by a R29 (First Award). The objective of this proposal is to characterize the mechanisms that underlie dynamic presynaptic changes in dopamine (DA)- and norepinephrine (NE)-containing afferents that occur during aging in brain areas known to regulate balance and gait. This work uses Fischer 344 rats as an animal model to assess age-related changes in dopaminergic inputs to the striatum and nucleus accumbens, as well as noradrenergic afferents to the cerebellum in intact animals. In addition, young and aged in oculo transplants of both substantia nigra and locus coeruleus are studied. These brain areas are known to be critically involved in movement. First, in vivo electrochemical recordings in anesthetized rats, using monoamine selective Nafion-coated electrodes and high-speed chronoamperometric recordings (5-25Hz), will be employed to quantitate the magnitude and high-resolution temporal characteristics of monoamine release in young and aged animals. In particular, amphetamine-and n-methyl-d-aspartate (NMDA)-induced release as compared to potassium-evoked release will be explored to investigate different types of presynaptic release processes in aging. The diffusion/clearance properties of locally-applied DA will also be investigated to more directly explore potential changes in high-affinity neuronal uptake and diffusion of DA which may occur with senescence. In addition, some measurements will be carried out in unrestrained freely- moving animals in order to investigate potential anesthetic effects associated with age-induced changes. Intracranial microdialysis experiments will be employed utilizing elevated dialysate levels of potassium, NMDA, and amphetamine to further substantiate the validity of the in vivo electrochemical protocols, and to explore potential changes in monoamine metabolites in the aged brain. Balance, coordination and motor learning tests will be performed in 14-16 month old animals and potassium-evoked overflow of DA will be tested in these animals to investigate if behavioral tests correlate with alterations in DA overflow. Secondly, potassium-and amphetamine-evoked overflow of NE will be carried out in the cerebellum of the different age groups analogous to the studies of DA to explore if NE afferents are changed in aging. Ne diffusion/clearance will be explored in the different age groups to more directly investigate high-affinity neuronal uptake processes in aging. Balance, coordination and motor learning tests will be performed in 14-16 month old animals to investigate if behavioral changes are correlated with alterations in potassium-evoked NE overflow. Finally, in oculo monoamine-containing brain cell transplants will be used as a model system to explore release processes from both young and aged locus coeruleus and substantia nigra brain grafts, and to investigate potential presynaptic changes that are extrinsically-versus intrinsically determined in aging.